1. Field of the Invention
This invention relates to an optical pickup apparatus for detecting (reading out) signals recorded on an optical recording medium such as an optical disk. This invention particularly relates to an optical pickup apparatus using an optical waveguide or a grating lens.
2. Description of the Prior Art
In recent years, optical recording media such as optical disks are widely used as recording media for image signals, audio signals or the like. Signals recorded on the optical recording medium in the form of pits, differences in reflectivity, or the like are detected by use of an optical pickup apparatus. (The case where signals are recorded using pits is hereinafter taken as an example.) The optical pickup apparatus is constituted to expose the surface of the optical recording medium to light such as a laser beam, and to detect the level of the light reflected by the optical recording medium, thereby to detect the presence or absence of a pit on the surface of the optical recording medium.
Besides the function of carrying out detection of the recorded signals as mentioned above, the optical pickup apparatus is required to have the function of detecting a tracking error, i.e. the function of detecting whether the light beam for pit detection is deviated rightward or leftward from the center of a predetermined pit string (track), and the function of detecting a focusing error, i.e. the function of detecting whether the focusing point of the light beam is deviated forward or backward from the reflection surface of the optical recording medium. A signal representing the tracking error or the focusing error is utilized to control tracking or focusing so that the signal is canceled, thereby to irradiate the light beam accurately to the predetermined track or to focus the light beam accurately onto the reflection surface of the optical recording medium. As the method of detecting a tracking error, there have heretofore been known the push-pull technique, the heterodyning technique, temporal (time difference) detection technique and other such techniques. On the other hand, as the method of detecting a focusing error, there have heretofore been known the astigmatism technique, the critical angle detection technique, the Foucault technique and other such techniques.
In order to provide the aforesaid functions, the conventional optical pickup apparatus is composed of small optical elements such as a beam splitter for splitting a light beam reflected by an optical recording medium from the light beam irradiated onto the optical recording medium, a lens for converging the reflected light beam in the vicinity of a photodetector such as a photodiode, and a prism for carrying out the tracking error detecting method and the focusing error detecting method.
However, the aforesaid small optical elements require accurate machining or processing, and troublesome adjustment of positions of the small optical elements with respect to each another must be carried out at the time of assembling the optical pickup apparatus. Therefore, the optical pickup apparatus using such optical elements naturally becomes expensive. Also, the optical pickup apparatus having the configuration as mentioned above is large and heavy, and therefore is disadvantageous from the viewpoint of making the read-out apparatus small and light and shortening the access time.
Accordingly, various attempts have heretofore been made to eliminate the aforesaid drawbacks of the conventional optical pickup apparatus by simplifying the configuration of the optical pickup apparatus by use of, for example, a special optical element such as an aspherical lens. However, the optical element of this type is very expensive, and therefore the cost of the optical pickup apparatus using such an optical element does not become much lower than the cost of the aforesaid optical pickup apparatus even though the configuration is simplified.